Optical loss of a silicon solar cell is mainly incurred through three approaches. The first is surface reflection of a silicon wafer, and a current solution is to manufacture a rough structure and an antireflection film on the front side to reduce the reflectance. The second is reflection and absorption by the backside surface. The silicon is poor at absorbing long-wavelength light, so that long-wavelength light at the aluminium backside surface is absorbed and reflected.
The third is loss of incident light caused by gridline shading. The reason is as follows. Generally, in order to enable the silicon solar cell to effectively collect excited carriers to form a photocurrent, electrodes are screen printed on the front and reverse sides of a substrate by using silver paste and silver aluminium paste respectively, so as to collect the carriers. Interdigitated electrodes, which are screen printed by using the silver paste and employed by the front side, block the incident light from entering the solar cell. The light arriving at the electrodes is directly reflected back or absorbed by the electrodes, and only small part of the light can undergo total internal reflection by a package material due to large angles and enter the solar cell to be absorbed.
In order to reduce the area blocked by the electrodes, currently two methods are employed. The first is to change the shape and size of the electrodes. Although the blocked area can be effectively reduced by downsizing the shape and decreasing the size of the electrodes, the minimum line width capable of being printed by the current screen printing has to be taken into consideration. The second is to change the overall architecture of the cell, where electrodes on the front sides are connected to the reverse side to eliminate the gridline shading problem on the front side, for example, the Emitter Wrap-Through (EWT) solar cell, the Metallization Wrap-Through (MWT) solar cell, and the Interdigitated Back Contact (IBC) solar cell. However the solar cells of these kinds require complex manufacturing processes, thereby making mass production difficult.
In U.S. Pat. No. 4,711,972, U.S. Pat. No. 5,110,370 and U.S. Pat. No. 5,228,926, a light directing structure is manufactured on an interface between a package material and the air, so that by taking advantages of a difference between the index of refraction of the package material and that of the air, incident light, which used to directly enter the electrodes, arrives in unblocked areas by refraction, thereby increasing the amount of the incident light. However, the structure is on the interface between the package material and air, so that when the cell module is placed outdoors, dust and stains are incurred, and it is easy for the dust to deposit on the manufactured structure, thereby decreasing the overall flux of the incident light.
In addition, in U.S. Pat. No. 5,076,857 and U.S. Pat. No. 5,554,229, a light guiding structure is manufactured on a contact surface of the cell, so as to enable light to undergo total reflection. However, the design incurs the contact with the solar cell, so that the cell may be electrically affected.